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Solubilization versus microemulsification of extractant molecules in micellar systems: Comparison between 8-hydroxyquinoline and Kelex 100
Solubilization versus Microemulsification of Extractant Molecules in Micellar Systems: Comparison between 8-Hydroxyquinoline and Kelex 100 M. BOUMEZIOUD, A. DEROUICHE, AND C. TONDRE 1 Laboratoire d'Etude des Solutions Organiques et Collofdales (LESOC), Unit~ AssociEe au CNRS No. 406, Universit3 de Nancy L B.P. 239, 54506 Vandoeuvre-lds-Nancy Cedex, France Received December 22, 1987; accepted May 16, 1988 We demonstrate from solubility measurements and from phase diagram determinations that the industrial extractant Kelex 100 can replace the oil component in a classical microemulsion system, thus leading to an isotropic dispersion of this complexing agent in water. This behavior, described here for the first time, differs significantly from the simple solubilization m e c h a n i s m encountered with its nonalkylated analog, 8-hydroxyquinoline. The potential importance of the results in metal recovery processes is discussed. © 1989AcademicPress.Inc.
INTRODUCTION
Much work has been devoted to the study of solubilization of all kinds of substances in micellar phases (1). The process of microemulsification itself can be considered a very special ease of solubilization: instead of being molecularly dispersed in a "solvent" phase, the solubilizate is dispersed in clusters of colloidal size. The solubilization in the first sense can never be expected to be greater in the hydrocarbon core of a micelle than in a hydrocarbon solvent (2). As far as ionic surfaetants are concerned, it is generally observed that a cosurfactant is necessary for microemulsification to occur. In recent years, microemulsion systems have been shown to have interesting applications in the field of metal recovery by solvent extraction processes. They can improve the kinetics of extraction (3-7), they can eoncentrate dilute aqueous solutions of metallic eations (8), and they are also useful for investigating the mechanisms of interracial reactions under conditions where the system, although microheterogeneous, has a homogeneous I TO w h o m correspondence should be addressed.
character (9). The methods applicable to homogeneous solutions can then be used to study the metal-extractant complexation reaction, whose kinetics is often thought to be the ratelimiting step in the extraction process (10). The influence of additives, the chemical nature of the extractant, or the medium itself can be easily studied that way. The solubilizing properties of extractant molecules in microemulsion systems may constitute an important factor for consideration in hydrometallurgical processes. For our part we have recently carried out stopped-flow experiments aimed at comparing the kinetics and mechanism ofcomplexation of Ni 2+ with 8-hydroxyquinoline (HQ) and of its alkylated derivative 7- (4-ethyl- 1-methyloctyl)-8-hydroxyquinoline ( C I I - H Q ) i n microemulsion phases (9). The latter compound is the active species of the industrial extractant Kelex 100 (11-19). The interpretation of our kinetic data required a knowledge of the distribution of the extractant molecules among the continuous aqueous phase, the dispersed oil phase, and the surfactant membrane, or at least a knowledge of their preferred location. The solubility measurements that we have performed for this
422 0021-9797/89 $3.00 Copyright© 1989by AcademicPress,Inc. All rightsof reproductionin any formreserved.
Journal ofColloidand InterfaceScience. Vol. 128,No. 2, March 15, 1989
423
EXTRACTANT MOLECULES IN MICELLES
purpose have shown some unexpected features which are reported here. EXPERIMENTAL
Materials. 8-Quinolinol (puriss) was obtained from Fluka and used without further purification. Kelex 100 was from Schering (FRG). The purification procedure used to obtain 7- (4-ethyl- 1-methyloctyl)-8-hydroxyquinoline has been described elsewhere (20). Recrystallized 99% sodium dodecyl sulfate (SDS) was obtained from Roth (FRG), 1pentanol puriss, and dodecane purum from Fluka. Solubilization measurements. The solubility of the extractant molecules in the different media was measured at saturation according to the following procedure. A known volume of solution was agitated for 3 days with an excess of the extractant. The solution was buffered at pH 7.5 with 0. l M triethanolamine/ HC104 to ensure that the extractant is essentially in its neutral form. (The pK's have been reported to be 5.1 and 9.9 for the nonalkylated extractant (21).) The concentration of solubilized extractant has been obtained from spectrophotometric measurements, taking the extinction coefficients given in Table I. The solutions were diluted in order to bring the
absorbance in the range where the Beer-Lambert law is satisfied. The general shape of the spectra was only weakly affected by the nature of the medium considered. The wavelength at maximum absorbance ~'maxis indicated in Table I. Two maxima are observed for the alkylated extractant. All the measurements were done at room temperature. RESULTS AND DISCUSSION
The results of the solubility measurements are reported in Fig. 1 for the two extractants. The solubility was first measured in water, and then SDS was added (micellar solution) up to 5.9% in weight. The addition of pentanol to the preceding solution and finally that of dodecane were performed so that the final solution obtained contained 5.9% SDS, 11.8% 1-pentanol, 6.8% dodecane, and 75.5% water. This corresponds to an oil-in-water microemulsion in the well-known pseudo-ternary diagram at a fixed surfactant (1/3 )/cosurfactant (2 / 3 ) weight ratio (22). For certain compositions gel-like solutions in which the solubility could not be measured were obtained. The important features in these measurements are the following:
TABLE I Effect of the Medium on the Extinction Coefficient (emax)and Wavelength at Maximum Absorption (Xmax) for 8-Hydroxyquinoline (HQ) and 7-(4-Ethyl-l-methyloctyl)-8-hydroxyquinoline ( C ~ - H Q ) HQ Medium
H20 pH 7 (TEA buffer) H20 + 1% 1-pentanol Micellar solution (SDS 2%) Micellar solution (SDS 5.9%) Microemulsion 5.9% SDS 11.8% l-pentanol 6.8% n-dodecane 75.5% water Pure l-pentanol Pure n-dodecane
~
(M -~. cm -~)
CII-HQ ~.~ (nm)
~
(M -x- cm -~)
) ~ (nm)
~2.~ (nm)
2.5 X 103 --2.1 X 103 2.7 X 103
307.7 304.7 307.7 307.7 309.4
----2.1 X 103
---312 315.6
---Vanishes 321.8
2.4 X 103 2.6 X 103
317.2 318.8
2.2 X 103 1.9 X 103
315.6 318.8
321.8 331.2
Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
424
B O U M E Z I O U D , DEROUICHE, A N D T O N D R E Solubitity
(MItI:,'L03
2®
h
I2~2!(kT.I
Ji!ili:ir --
li!;!::)l tt.gl
add.SDSadd.pentcmot add.dodecane FIG. 1. Solubifity of the extractants at saturation, in moles per liter of the system considered. Zero abscissa gives the solubility in pure water. The first section of the solubility curves shows the effect of addition of SDS. The second section shows the effect of adding l-pentanol to the preceding final solution. The third section is relative to the addition of dodecane to the mixed alcohol + surfactant solution. The concentrations of additives are given in weight percent.
- - F o r 8-hydroxyquinoline (HQ), the solubility is much larger in pure micellar solutions of SDS than in either pure water or pure dodecane. It does not improve very much by addition of pentanol (in fact it decreases first) and dodecane. - - F o r 7- ( 4-ethyl- 1-methyloctyl ) -8-hydroxyquinoline ( C l r H Q ) , there is no solubility at all in water, as expected, and a very low solubility in pure SDS micelles. As soon as traces of 1-pentanol are added the solubility starts to increase and becomes very large, but it diminishes with the addition of dodecane. These results can be understood if one assumes that a classical solubilization process takes place for the first extractant and a process of microemulsification for the alkylated one. Indeed the m i n i m u m of solubility observed for HQ when adding 1-pentanol can be easily explained when considering the previously reported change of aggregation number in mixed micelles of SDS and pentanol. A minimum of Journal of Colloid and Interface Science, Vol. 128,No. 2, March 15, 1989
the aggregation number has been observed at about 0.45 M of pentanol in SDS micellar solutions (23), i.e., precisely where the minimum of solubility appears. On the other hand, the relation between the solubilizing power of micelles and their aggregation number is well known (24). In the case of C11-HQ, when the solubility data are extrapolated at 11.8% of l-pentanol it is found that approximately 0.23 mole of extractant can be incorporated in 1 liter of the system. This corresponds to about 7% in weight, which is of the same order of magnitude as the amount of dodecane that can be isotropically incorporated in the same system in the absence of extractant. It is thus concluded that CI~-HQ plays exactly the same part as an oil and the decrease in solubility observed in Fig. 1, when dodecane is added, is the result of a competition between these two oils for solubilization. Assuming a pseudo-phase model (25) for the final microemulsion, we can deduce from these data that the extractant HQ is preferentially solubilized in the amphiphilic membrane pseudo-phase of the microemulsion droplets. This is confirmed by the value of ~max (Table I): a bathochromic shift is observed between water and dodecane, due to the change of environment; the value in the microemulsion is intermediate between those in water and those in dodecane, and close to that in micellar solutions of SDS. For the extractant C~ ~-HQ the value of Xmax in the microemulsion is intermediate between the values in a micellar solution and those in dodecane, suggesting a partitioning of the extractant between the amphiphilic membrane and the dodecane core. In order to have a definitive confirmation of the microemulsification of the alkylated extractant, we have established the pseudo-ternary-phase diagram of the system SDS ( 1/ 3 ) 1-pentanol (2 / 3) / water/Kelex 100, where the commercial extractant replaces the usual oil. The limits of the monophasic domain are shown in Fig. 2, confirming the existence of a large microemulsion domain in the total ab-
EXTRACTANT MOLECULES IN MICELLES
2,3
7 H20
20
...... 40
60
7?\ 80
KELEX100or dodecone--
FIG. 2. Pseudo-ternary-phase diagram of the system water / SDS ( 1/ 3 )- 1-pentanol ( 2 / 3 )/ Kelex 100 in weight percent. The dashed lines show for comparison the phase limits for the system including dodecane in place of Kelex 100. sence o f oil. T h e c o m p a r i s o n with the corresponding diagram with dodecane shows a reduction o f the area o f the monophasic domain. We will n o w discuss the potential use o f these microemulsions o f Kelex 100. The first observation concerns the problem o f chemical reactivity in microemulsion systems. W h e n such systems are used to improve the rate, the yield, the selectivity, etc., o f a particular chemical reaction, it m a y be advantageous if one o f the reactants is a c o m p o n e n t o f the microemulsion system itself. This is precisely the case here if we are interested in the reactions o f complexation o f Kelex 100 with metallic species. The microemulsion can be considered a dispersion o f microscopic chelating particles in a water c o n t i n u u m , comparable in some way to the beads o f a chelating resin, except that it is not solid in nature. The use o f such microemulsions for metal extraction could provide us with new processes in which there would be no m o r e need for an organic solvent. More work will be necessary to investigate the phase behavior o f these microemulsion systems when parameters such as the pH, the salinity o f water, or the temperature are changed. A g o o d knowledge o f the fie-lines along which phase separations occur would also be very helpful in this regard.
425
O n e can hope that conditions will be f o u n d u n d e r which the addition to the microemulsion o f Kelex o f an aqueous solution containing metal ions would lead to a biphasic system with the K e l e x - m e t a l complex floating on an aqueous phase (equivalent to a so-called Winsor I system). O n the other hand, it would also be interesting to carry out experiments aimed at seeing whether the present observation has some kind o f general character. O n e can think o f two different kinds o f investigations: (i) microemulsification o f Kelex by other surfactants ( a n d eventually cosurfactant), particularly o f low economical cost; (ii) microemulsification o f other highly lipophilic ligands having an oily quality. We are pursuing our research in this field in order to try to answer the preceding questions. REFERENCES 1. "Micellisation, Solubilization and Microemulsions" (K. L. Mittal, Ed.), Vol. 1, 2. Plenum, New York, 1977; "Solution Chemistry ofSurfactants," (K. L. Mittal, Ed.), Vol. 1, 2. Plenum, New York, 1979; "Surfactants in Solution" (K. L. Mittal and B. Lindman, Eds.), Vol. 1-3. Plenum, New York, 1984; (K. L. Mittal and P. Bothorel, Eds.), Vol. 4-6. Plenum, New York, 1986. 2. Almgren, M., Grieser, F., and Thomas, J. K., J. Amer. Chem. Soc. 101, 279 (1979). 3. Bauer, D., and Komornicki, J., "ISEC 1983, Denver," Interfacial Kinetics, Vol. II, p. 315. 4. Fourre, P., and Bauer, D., C.R. Acad. Sci. Paris Ser. H292, 1077 (1981). 5. Wu, C. K., et aL, Sci. Sin. 23, 1533 (1980); "ISEC 1980, Libge," Vol. 3, Dispersion 80-23. 6. Osseo-Asare,K., and Keeney, M. E., Sep. Sci. Technol. 15, 999 (1980). 7. Tondre, C., and Xenakis, A., Faraday Discuss. Chem. Soc. 77, 115 (1984). 8. Ovejero-Escudero,F. J., Angelino, H., and Casamatta, G., J. Disp. Sci. Technol. 8, 89 (1987). 9. Tondre, C., and Boumezioud, M., J. Phys. Chem., in press. 10. Danesi, P., and Chiarizia, R., CRC Crit. Rev. Anal Chem. 10(1), 1 (1980). 11. Hartlage, J. A., Soc. Mining Engrs Meeting (AIME), Salt Lake City, 1969. 12. Ashbrook, A. W., Coord. Chem. Rev. 16, 285 (1975). 13. Leveque, A., and Helgorsky, J., "Proceedings, Int. Solv. Extr. Conf., ISEC 1977," CIM special volume, Vol. 21,p. 439. Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
426
BOUMEZIOUD, DEROUICHE, AND TONDRE
14. Demopoulos, G. P., and Distin, P. A., Hydrometallurgy 11, 389 (1983). 15. Cote, G., and Bauer, D., Hydrometallurgy 5, 149 (1980); Bauer, D., Fourre, P., and Lemerle, J., C,R. Acad. Sci. Paris Ser. H 292, 1019 (1981); Fourre, P., Bauer, D., and Lemerle, J., Anal Chem. 55, 662 (1983 ); Marchon, B., Cote, G., and Bauer, D., J. Inorg. Nucl. Chem. 41, 1353 (1979); Guesnet, P., Sabot, J. L., and Bauer, D., J. Inorg. Nucl. Chem. 42, 1459 (1980). 16. Lakshmanan, V. I., and Lawson, G. J., J. Inorg. Nucl. Chem. 35, 4285 (1973). 17. Flett, D. S., Lox, M., and Heels, J. D., J. Inorg. Nucl. Chem. 37, 2197 (1975); Flett, D. S., Hartlage, J. A., Spink, D. R., and Okuhara, D. N., J. Inorg. Nucl. Chem. 37, 1967 (1975). 18. Ritcey, G. M., and Lucas, B. H., CIM Bull. 64, 87 (1974); 68, 105 (1975).
Journalof ColloidandInterfaceScience,Vol.128,No. 2, March15, 1989
19. Haraguchi, K., and Freiser, H., Inorg. Chem. 22, 1187 (1983); Bag, S. T., and Freiser, H., Anal, Chim. Acta 135, 319 (1982); Zhu, L., and Freiser, H., Anal, Chim. Acta 146, 237 (1983). 20. Boumezioud, M., Lagrange, P., and Tondre, C., Polyhedron 7, 513 (1988). 21. Johnson, W. A., and Wilkins, R. G., Inorg. Chem. 9, 1917 (1970). 22. Tondre, C., and Zana, R., J. Dispersion Sci. Technol. 1, 179 (1980). 23. Lianos, P., Lang, J., Strazielle, C., and Zana, R., J. Phys. Chem. 86, 1019 (1982). 24. Mukerjee, P., in "Solution Chemistry of Surfactants" (K. L. Mittal, Ed.), Vol. I, p. 153. Plenum, New York, 1979. 25. Biais, J., Bothorel, P., Clin, B., and Lalanne, P., J. Dispersion Sci. Technol. 2, 67 (1981).
INTRODUCTION
Much work has been devoted to the study of solubilization of all kinds of substances in micellar phases (1). The process of microemulsification itself can be considered a very special ease of solubilization: instead of being molecularly dispersed in a "solvent" phase, the solubilizate is dispersed in clusters of colloidal size. The solubilization in the first sense can never be expected to be greater in the hydrocarbon core of a micelle than in a hydrocarbon solvent (2). As far as ionic surfaetants are concerned, it is generally observed that a cosurfactant is necessary for microemulsification to occur. In recent years, microemulsion systems have been shown to have interesting applications in the field of metal recovery by solvent extraction processes. They can improve the kinetics of extraction (3-7), they can eoncentrate dilute aqueous solutions of metallic eations (8), and they are also useful for investigating the mechanisms of interracial reactions under conditions where the system, although microheterogeneous, has a homogeneous I TO w h o m correspondence should be addressed.
character (9). The methods applicable to homogeneous solutions can then be used to study the metal-extractant complexation reaction, whose kinetics is often thought to be the ratelimiting step in the extraction process (10). The influence of additives, the chemical nature of the extractant, or the medium itself can be easily studied that way. The solubilizing properties of extractant molecules in microemulsion systems may constitute an important factor for consideration in hydrometallurgical processes. For our part we have recently carried out stopped-flow experiments aimed at comparing the kinetics and mechanism ofcomplexation of Ni 2+ with 8-hydroxyquinoline (HQ) and of its alkylated derivative 7- (4-ethyl- 1-methyloctyl)-8-hydroxyquinoline ( C I I - H Q ) i n microemulsion phases (9). The latter compound is the active species of the industrial extractant Kelex 100 (11-19). The interpretation of our kinetic data required a knowledge of the distribution of the extractant molecules among the continuous aqueous phase, the dispersed oil phase, and the surfactant membrane, or at least a knowledge of their preferred location. The solubility measurements that we have performed for this
422 0021-9797/89 $3.00 Copyright© 1989by AcademicPress,Inc. All rightsof reproductionin any formreserved.
Journal ofColloidand InterfaceScience. Vol. 128,No. 2, March 15, 1989
423
EXTRACTANT MOLECULES IN MICELLES
purpose have shown some unexpected features which are reported here. EXPERIMENTAL
Materials. 8-Quinolinol (puriss) was obtained from Fluka and used without further purification. Kelex 100 was from Schering (FRG). The purification procedure used to obtain 7- (4-ethyl- 1-methyloctyl)-8-hydroxyquinoline has been described elsewhere (20). Recrystallized 99% sodium dodecyl sulfate (SDS) was obtained from Roth (FRG), 1pentanol puriss, and dodecane purum from Fluka. Solubilization measurements. The solubility of the extractant molecules in the different media was measured at saturation according to the following procedure. A known volume of solution was agitated for 3 days with an excess of the extractant. The solution was buffered at pH 7.5 with 0. l M triethanolamine/ HC104 to ensure that the extractant is essentially in its neutral form. (The pK's have been reported to be 5.1 and 9.9 for the nonalkylated extractant (21).) The concentration of solubilized extractant has been obtained from spectrophotometric measurements, taking the extinction coefficients given in Table I. The solutions were diluted in order to bring the
absorbance in the range where the Beer-Lambert law is satisfied. The general shape of the spectra was only weakly affected by the nature of the medium considered. The wavelength at maximum absorbance ~'maxis indicated in Table I. Two maxima are observed for the alkylated extractant. All the measurements were done at room temperature. RESULTS AND DISCUSSION
The results of the solubility measurements are reported in Fig. 1 for the two extractants. The solubility was first measured in water, and then SDS was added (micellar solution) up to 5.9% in weight. The addition of pentanol to the preceding solution and finally that of dodecane were performed so that the final solution obtained contained 5.9% SDS, 11.8% 1-pentanol, 6.8% dodecane, and 75.5% water. This corresponds to an oil-in-water microemulsion in the well-known pseudo-ternary diagram at a fixed surfactant (1/3 )/cosurfactant (2 / 3 ) weight ratio (22). For certain compositions gel-like solutions in which the solubility could not be measured were obtained. The important features in these measurements are the following:
TABLE I Effect of the Medium on the Extinction Coefficient (emax)and Wavelength at Maximum Absorption (Xmax) for 8-Hydroxyquinoline (HQ) and 7-(4-Ethyl-l-methyloctyl)-8-hydroxyquinoline ( C ~ - H Q ) HQ Medium
H20 pH 7 (TEA buffer) H20 + 1% 1-pentanol Micellar solution (SDS 2%) Micellar solution (SDS 5.9%) Microemulsion 5.9% SDS 11.8% l-pentanol 6.8% n-dodecane 75.5% water Pure l-pentanol Pure n-dodecane
~
(M -~. cm -~)
CII-HQ ~.~ (nm)
~
(M -x- cm -~)
) ~ (nm)
~2.~ (nm)
2.5 X 103 --2.1 X 103 2.7 X 103
307.7 304.7 307.7 307.7 309.4
----2.1 X 103
---312 315.6
---Vanishes 321.8
2.4 X 103 2.6 X 103
317.2 318.8
2.2 X 103 1.9 X 103
315.6 318.8
321.8 331.2
Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
424
B O U M E Z I O U D , DEROUICHE, A N D T O N D R E Solubitity
(MItI:,'L03
2®
h
I2~2!(kT.I
Ji!ili:ir --
li!;!::)l tt.gl
add.SDSadd.pentcmot add.dodecane FIG. 1. Solubifity of the extractants at saturation, in moles per liter of the system considered. Zero abscissa gives the solubility in pure water. The first section of the solubility curves shows the effect of addition of SDS. The second section shows the effect of adding l-pentanol to the preceding final solution. The third section is relative to the addition of dodecane to the mixed alcohol + surfactant solution. The concentrations of additives are given in weight percent.
- - F o r 8-hydroxyquinoline (HQ), the solubility is much larger in pure micellar solutions of SDS than in either pure water or pure dodecane. It does not improve very much by addition of pentanol (in fact it decreases first) and dodecane. - - F o r 7- ( 4-ethyl- 1-methyloctyl ) -8-hydroxyquinoline ( C l r H Q ) , there is no solubility at all in water, as expected, and a very low solubility in pure SDS micelles. As soon as traces of 1-pentanol are added the solubility starts to increase and becomes very large, but it diminishes with the addition of dodecane. These results can be understood if one assumes that a classical solubilization process takes place for the first extractant and a process of microemulsification for the alkylated one. Indeed the m i n i m u m of solubility observed for HQ when adding 1-pentanol can be easily explained when considering the previously reported change of aggregation number in mixed micelles of SDS and pentanol. A minimum of Journal of Colloid and Interface Science, Vol. 128,No. 2, March 15, 1989
the aggregation number has been observed at about 0.45 M of pentanol in SDS micellar solutions (23), i.e., precisely where the minimum of solubility appears. On the other hand, the relation between the solubilizing power of micelles and their aggregation number is well known (24). In the case of C11-HQ, when the solubility data are extrapolated at 11.8% of l-pentanol it is found that approximately 0.23 mole of extractant can be incorporated in 1 liter of the system. This corresponds to about 7% in weight, which is of the same order of magnitude as the amount of dodecane that can be isotropically incorporated in the same system in the absence of extractant. It is thus concluded that CI~-HQ plays exactly the same part as an oil and the decrease in solubility observed in Fig. 1, when dodecane is added, is the result of a competition between these two oils for solubilization. Assuming a pseudo-phase model (25) for the final microemulsion, we can deduce from these data that the extractant HQ is preferentially solubilized in the amphiphilic membrane pseudo-phase of the microemulsion droplets. This is confirmed by the value of ~max (Table I): a bathochromic shift is observed between water and dodecane, due to the change of environment; the value in the microemulsion is intermediate between those in water and those in dodecane, and close to that in micellar solutions of SDS. For the extractant C~ ~-HQ the value of Xmax in the microemulsion is intermediate between the values in a micellar solution and those in dodecane, suggesting a partitioning of the extractant between the amphiphilic membrane and the dodecane core. In order to have a definitive confirmation of the microemulsification of the alkylated extractant, we have established the pseudo-ternary-phase diagram of the system SDS ( 1/ 3 ) 1-pentanol (2 / 3) / water/Kelex 100, where the commercial extractant replaces the usual oil. The limits of the monophasic domain are shown in Fig. 2, confirming the existence of a large microemulsion domain in the total ab-
EXTRACTANT MOLECULES IN MICELLES
2,3
7 H20
20
...... 40
60
7?\ 80
KELEX100or dodecone--
FIG. 2. Pseudo-ternary-phase diagram of the system water / SDS ( 1/ 3 )- 1-pentanol ( 2 / 3 )/ Kelex 100 in weight percent. The dashed lines show for comparison the phase limits for the system including dodecane in place of Kelex 100. sence o f oil. T h e c o m p a r i s o n with the corresponding diagram with dodecane shows a reduction o f the area o f the monophasic domain. We will n o w discuss the potential use o f these microemulsions o f Kelex 100. The first observation concerns the problem o f chemical reactivity in microemulsion systems. W h e n such systems are used to improve the rate, the yield, the selectivity, etc., o f a particular chemical reaction, it m a y be advantageous if one o f the reactants is a c o m p o n e n t o f the microemulsion system itself. This is precisely the case here if we are interested in the reactions o f complexation o f Kelex 100 with metallic species. The microemulsion can be considered a dispersion o f microscopic chelating particles in a water c o n t i n u u m , comparable in some way to the beads o f a chelating resin, except that it is not solid in nature. The use o f such microemulsions for metal extraction could provide us with new processes in which there would be no m o r e need for an organic solvent. More work will be necessary to investigate the phase behavior o f these microemulsion systems when parameters such as the pH, the salinity o f water, or the temperature are changed. A g o o d knowledge o f the fie-lines along which phase separations occur would also be very helpful in this regard.
425
O n e can hope that conditions will be f o u n d u n d e r which the addition to the microemulsion o f Kelex o f an aqueous solution containing metal ions would lead to a biphasic system with the K e l e x - m e t a l complex floating on an aqueous phase (equivalent to a so-called Winsor I system). O n the other hand, it would also be interesting to carry out experiments aimed at seeing whether the present observation has some kind o f general character. O n e can think o f two different kinds o f investigations: (i) microemulsification o f Kelex by other surfactants ( a n d eventually cosurfactant), particularly o f low economical cost; (ii) microemulsification o f other highly lipophilic ligands having an oily quality. We are pursuing our research in this field in order to try to answer the preceding questions. REFERENCES 1. "Micellisation, Solubilization and Microemulsions" (K. L. Mittal, Ed.), Vol. 1, 2. Plenum, New York, 1977; "Solution Chemistry ofSurfactants," (K. L. Mittal, Ed.), Vol. 1, 2. Plenum, New York, 1979; "Surfactants in Solution" (K. L. Mittal and B. Lindman, Eds.), Vol. 1-3. Plenum, New York, 1984; (K. L. Mittal and P. Bothorel, Eds.), Vol. 4-6. Plenum, New York, 1986. 2. Almgren, M., Grieser, F., and Thomas, J. K., J. Amer. Chem. Soc. 101, 279 (1979). 3. Bauer, D., and Komornicki, J., "ISEC 1983, Denver," Interfacial Kinetics, Vol. II, p. 315. 4. Fourre, P., and Bauer, D., C.R. Acad. Sci. Paris Ser. H292, 1077 (1981). 5. Wu, C. K., et aL, Sci. Sin. 23, 1533 (1980); "ISEC 1980, Libge," Vol. 3, Dispersion 80-23. 6. Osseo-Asare,K., and Keeney, M. E., Sep. Sci. Technol. 15, 999 (1980). 7. Tondre, C., and Xenakis, A., Faraday Discuss. Chem. Soc. 77, 115 (1984). 8. Ovejero-Escudero,F. J., Angelino, H., and Casamatta, G., J. Disp. Sci. Technol. 8, 89 (1987). 9. Tondre, C., and Boumezioud, M., J. Phys. Chem., in press. 10. Danesi, P., and Chiarizia, R., CRC Crit. Rev. Anal Chem. 10(1), 1 (1980). 11. Hartlage, J. A., Soc. Mining Engrs Meeting (AIME), Salt Lake City, 1969. 12. Ashbrook, A. W., Coord. Chem. Rev. 16, 285 (1975). 13. Leveque, A., and Helgorsky, J., "Proceedings, Int. Solv. Extr. Conf., ISEC 1977," CIM special volume, Vol. 21,p. 439. Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
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